The present invention relates generally to compositions for sealing and adhering magnesium-based substrates. More particularly, the present invention relates to compositions which demonstrate excellent resistance to fluid solvents, particularly automotive fluids, while maintaining excellent adherence to magnesium-based substrates.
Room temperature vulcanizable (RTV) silicones posses an interesting combination of properties that make them very desirable for a large number of applications. These properties include a viscosity range that allows for use as flowable liquids or soft pastes, excellent thermal stability characteristics of high-consistency silicone rubber, good adhesion to many surfaces without requiring pressure, and curing without the need for heating, as their name implies. However, RTV silicones have not been used for sealing and bonding magnesium-based substrates, such as magnesium alloy substrates.
It is known to add viscosity-modifying filler materials to RTV silicones to improve their resistance to fluid solvents. For example, the addition of certain grades of metal oxides to silicone elastomers is known to result in silicone rubber compositions having a certain degree of oil resistance. European Patent Publication No. 572 148, assigned to General Electric Company, discloses the incorporation of mixed metal oxides into heat cured silicone elastomeric compositions containing MQ resins (M=R3SiO1/2 monofunctional groups; Q=SiO2 quadri- functional groups). In the ""148 publication, such compositions are formed into engine gaskets which are reported to display a certain degree of oil resistance.
It is further known to add silane compounds to silicone elastomers to improve the adhesive properties of RTV silicone compositions. For example, U.S. Pat. No. 5,569,750 to Knepper et al. discloses a silicone rubber composition including an aminohydrocarbyl-substituted ketoximinosilane which reportedly possesses improved adhesive properties.
Silicone elastomers are commonly used as adhesives and sealants in automotive applications. For instance, U.S. Pat. Nos. 4,847,396 and 4,735,979, both assigned to Loctite Corporation, disclose RTV silicone compositions as automotive sealants. These patents disclose the use of adhesion promoters in silicone compositions to improve the adhesive strength of the silicone compositions to ferrous and aluminum substrates, even when these substrates have motor oil coated on their surfaces. Further, U.S. Pat. No. 5,434,214, also assigned to Loctite Corporation, discloses hydrosilation cured silicone formulations which cure at room temperature and, after a subsequent heat cure cycle, provide good primerless adhesion to aluminum and titanium substrates.
Magnesium alloys, typically alloys which include magnesium, aluminum and, to a lesser degree, zinc and manganese, have traditionally been used in aeronautic applications. They are characterized as being light in weight, offering high strength, being easily molded, having the ability to withstand high tolerances, and having the ability to withstand high temperatures. As a result of these properties, magnesium alloys are finding increased utility in other areas, such as automotive applications.
Magnesium alloys have traditionally been joined by riveting or welding. It is desirable, however, to bond and seal magnesium alloys using adhesives, as adhesive bonding and sealing offers advantages in weight saving, fatigue strength, and corrosion resistance. Attempts at such bonding and sealing, however, have proven generally commercially unacceptable for many applications, particularly automotive applications where the adhesives are exposed to automotive fluids, such as engine oils and coolants, at temperatures observed during operation of an automobile engine. These fluids tend to degrade the adhesives, thereby destroying their ability to seal and bond the substrates. Further, oxidation on the surface of magnesium alloys results in the formation of a layer of magnesium oxide which prevents bonding of adhesive materials to the underlying magnesium alloy.
Therefore, there exists a need for an elastomeric composition which exhibits improved adherence to magnesium-based substrates such as magnesium alloys, maintains structural integrity upon exposure to fluid solvents, and exhibits flexibility when subjected to thermal and mechanical stresses. The current invention is directed towards meeting these and other needs.
The present invention is directed to compositions which are suitable for use as sealants and adhesives on magnesium-based substrates such as magnesium alloy substrates. Particularly, the present invention is directed to a silicone composition, the reaction products of which demonstrate improved adhesion to magnesium-based substrates, which includes at least one polymerizable silicone component, at least one amino-containing silane adhesion promoter, and at least one filler. These compositions demonstrate excellent resistance to fluids, such as automotive fluids, while maintaining their enhanced adherence to magnesium-based substrates. Additionally, these compositions maintain their structural integrity when subjected to thermal and mechanical stresses. Desirably, the compositions include one or more polymerizable silicone polymers, such as RTV silicones, at least one amino-containing silane compound which serves as a magnesium alloy adhesion promoter, and at least one filler which serves to provide fluid resistance to the compositions. The present compositions can be used to seal interfaces between magnesium-based substrates, bond magnesium-based substrates together, and bond magnesium-based substrates to non-magnesium-based substrates. The present invention is also directed to a method for providing enhanced adhesion to magnesium-based substrates by disposing a composition of the present invention between two substrates and then curing the composition. Additionally, the present invention is directed to an article of manufacture including a composition of the present invention.
Compositions of the present invention can be dispensed on magnesium-based substrates in a liquid phase and allowed to cure at room temperature or can be cured prior to application to form automotive sealant devices, such as gaskets and o-rings. Methods of making these compositions and their use are also disclosed.
The present invention is directed to compositions which are curable to an elastomeric state and which exhibit improved adherence to magnesium-based substrates such as magnesium alloys and enhanced resistance to fluids, such as those commonly used in and around automotive engines. Compositions of the present invention include polymerizable silicone compounds, such as reactive polysiloxanes, at least one amino-containing silane compounds, and at least one filler.
Any conventional polysiloxane may be used in compositions of the present invention and may be present in any amount, provided the resulting compositions effectively bond to magnesium-based substrates, form effective seals when cured, and are rendered substantially insoluble or resistant to automotive fluid solvents. For example, room temperature curing, actinic radiation curing, heat curing or silicones which cure by more than one of these mechanisms may be used. Desirably, the polysiloxane is an RTV compound. RTV compounds are typically low-consistency silicone elastomers that can be easily extruded from a tube or other dispenser and subsequently cured. Silicones which cure at room temperature may exist in one-part systems or in two-part systems.
In a one-part system, the silicones are typically organosiloxane compounds which may include hydrolyzable groups on their ends. They are stored in a noncrosslinked state in a moisture-impermeable container until they are to be used. When they are removed from the container and exposed to moisture, the hydrolyzable groups react with the moisture in a condensation reaction, resulting in crosslinking of the polymer. An example of an RTV organopolysiloxane is a hydroxy-terminated diorganopolysiloxane represented by the formula: 
wherein m is from about 50 to about 2000, desirably from about 500 to about 800, R is independently an unsubstituted or substituted monovalent hydrocarbon group exemplified by alkyl groups, such as methyl, ethyl, propyl and butyl groups; cycloalkyl groups, such as a cyclohexyl group; alkenyl groups, such as vinyl and allyl groups; and aryl groups, such as phenyl and tolyl groups; as well as those substituted groups obtained by replacing a part or all of the hydrogen atoms in the above-referenced hydrocarbon groups with halogen atoms, cyano groups and the like.
Desirably, the polysiloxane used in the present invention is a hydroxy-terminated poly(dimethylsiloxane). This polysiloxane may also be a single hydroxy-terminated poly(dimethylsiloxane) or may be a combination of hydroxy-terminated poly(dimethylsiloxanes) which are of different viscosities. For example, they may have viscosities of from 2000 cps to 10,000 cps, correlating to a molecular weight range of approximately 36,000 to 60,000. Such hydroxy-terminated poly(dimethylsiloxane) compounds are available commercially from Union Carbide, identified by proprietary designations Y-7839 and Y-7869, and from Mobay, identified by proprietary designation Baylisone E-2.
RTV silicone compositions have long been used in automotive and other applications and many variations thereof are known. By way of example, RTV silicones have been formulated as automotive engine cured-after-assembly gasketing materials where high temperature aggressive fluids, such as engine coolant and motor oil, create a particularly hostile environment. Examples of silicones formulated for such automotive gasketing applications are described in U.S. Pat. Nos. 4,673,750 (Beers et al.), 4,735,979 (Beers et al.), 4,847,396 (Beers et al.), and 5,346,940 (Brassard et al.), all expressly incorporated by reference herein.
Useful heat curable siloxanes that may be used in the present invention include, but are not limited to, linear polyorganosiloxanes, monomeric siloxanes, polydimethylsiloxane chains having SiH functionality located at either the ends or the middle of the chains, cyclic siloxanes and combinations thereof. Particularly useful heat curable silicones include polysiloxanes in combination with vinyl-terminated low molecular weight polysiloxanes. More particularly, dimethyl vinyl-terminated polydimethylsiloxane (10,000 MW vinyl fluid) and vinyl-terminated polydimethylsiloxane (62,000 MW vinyl fluid) may be used as a heat curable polymer in the present invention.
Heat curable polyorganosiloxanes having olefinic unsaturation should contain at least one reactive functional group, and desirably two reactive functional groups. More than two reactive functional groups are also contemplated. The number and type of functional group or groups present can be varied according to the desired properties of the final silicone composition. The organic groups of the polyorganosiloxane are monovalent hydrocarbon radicals and preferably the organo groups are selected from alkyl radicals, such as methyl, ethyl, propyl, etc.; alkenyl radicals such as vinyl, allyl, etc.; cycloalkyl radicals such as cyclohexyl, cycloheptyl, and; mononuclear aryl radicals such as phenyl, ethylphenyl; and haloalkyl such as 3, 3, 3-trifluoropropyl.
Heat curable polyorganosiloxanes useful in the present invention are represented by the following general formula: 
wherein n is an integer such that the viscosity is from about 25 cps to about 2,500,000 cps at 25xc2x0 C., such as when n is from 1 to 1,200 and desirably from 10 to 1,000; R1, R2, R3 and R5 can be the same or different and are substituted or unsubstituted hydrocarbon or hydrocarbonoxy radicals from C1-20, provided that at least one of these R groups, and desirably more than one, are selected from the reactive functional groups consisting of (meth)acrylate, carboxylate, maleate, cinnamate and combinations thereof, and provided that the reactive functional group is not directly bonded to a silicon atom, but separated from the silicon atom by an intervening chemical moiety, such as an atom or chemical group. For example, when one or more of the aforementioned R groups (R1, R2, R3 and R5) is not one of the required reactive functional groups, they can be chosen from alkyl radicals such as methyl, ethyl, propyl, butyl and pentyl; alkenyl radicals such as vinyl and allyl; cycloalkyl radicals such as cyclohexyl and cycloheptyl; aryl radicals such as phenyl, methylphenyl, ethylphenyl; arylalkyl radicals such as beta-phenylethyl; alkylaryl radicals; and hydrocarbonoxy radicals such as alkoxy, aryloxy, alkaryloxy, aryalkoxy, and desirably methoxy, ethoxy or hydroxy, and the like. Any of the foregoing radicals may have some or all of the hydrogen atoms replaced, for example, by a halogen such as fluorine or chlorine. One or more of the aforementioned R groups can also be hydrogen, provided the required reactive functional group is present as indicated and the presence of the hydrogen does not deleteriously interfere with the ability of the polyorganosiloxane to perform in the present invention. R3 in the above formula desirably is a vinyl group or a dimethyl vinyl group.
Radiation curable silicones, particularly those curable by ultraviolet light (UV) and electron beam (EB), may also be used in the present invention. Organopolysiloxane polymers are rendered radiation hardenable by functionalizing them with radiation susceptible groups. The three most common groups used to convert silicones as radiation curable are mercapto-olefins, epoxies, and acrylics. For instance, acrylics and mercapto-olefins crosslink by free radicals and, therefore, can be crosslinked in the presence of suitable photoinitiators. One example of a UV curable silicone composition useful in the present invention is described in U.S. Pat. No. 5,300,608 (Chu), the disclosure of which is expressly incorporated herein by reference. The photocurable silicone composition is described therein to include alkoxy-terminated organopolysiloxanes represented by the formula: 
wherein R1, R2, R3 and R4 may be identical or different and are monovalent hydrocarbon radicals having up to 10 carbon atoms (C1-10), or halo or cyano substituted hydrocarbon radicals; R3 may also be a monovalent heterohydrocarbon radical having up to 10 carbon atoms (C1-10), wherein the heteroatoms are selected from the group consisting of haloatoms, O, N; R5 is alkyl (C1-C10), preferably methyl, ethyl or isopropyl; R5 may also be a CH2CH2OCH3; n is an integer; a is 0, 1 or 2; b is 0, 1 or 2; and a+b is 0, 1 or 2. In a particularly desirable embodiment, R3 is a methacryloxypropyl group, R4 and R5 are methyl groups, and R1 and R2 are methyl groups. The alkoxy-terminated organopolysiloxanes can be prepared as described in the ""608 patent.
It is also known to cure polymerizable silicone compounds by more than one curing method. For example, U.S. Pat. No. 5,212,211 (Welch II et al.), expressly incorporated by reference herein, discloses a silicone compound which is curable by more than one method of curing, such as heat curing, moisture curing, or curing with ultraviolet light. U.S. Pat. No. 5,516,812 (Chu et al.), expressly incorporated by reference herein, discloses a silicone composition which is both UV and moisture cured. Additionally, European Patent 539 234 discloses a composition for liquid gasketing which has both an UV curing property and a moisture-curing property and Japanese Patent Application No. 92143102 to Tokyo Three Bond Co., Ltd. discloses compositions which are both moisture curable and UV curable.